KR20220055270A - Steel sheet having ultra high strength with high formability and method of manufacturing the same - Google Patents

Abstract

Provided in the present invention are an ultra-high strength and high-formability steel plate and a manufacturing method thereof. According to one embodiment of the present invention, the ultra-high strength and high-formability steel plate comprises: 0.10-0.22 wt% of carbon (C); 0.1-1.0 wt% of silicon (Si); 2.3-2.7 wt% of manganese (Mn); 0.01-0.10 wt% of soluble aluminum (S_Al); 0.1-1.0 wt% of chromium (Cr); 0.01-0.05 wt% of niobium (Nb); 0.001-0.05 wt% of titanium (Ti); 0.0001-0.004 wt% of boron (B); more than 0 wt% and equal to or less than 0.02 wt% of phosphorus (P); more than 0 wt% and equal to or less than 0.003 wt% of sulfur (S); more than 0 wt% and equal to or less than 0.006 wt% of nitrogen (N); and the balance of iron (Fe) and other inevitable impurities. The present invention has a mixed structure in which ferrite, bainite and martensite are mixed. The fraction of ferrite is within 1-10 %, and satisfies yield strength (YS) equal to or more than 900 MPa, tensile strength (TS) equal to or more than 1180 MPa, elongation (EL) equal to or more than 5, the yield ratio (YR) equal to or more than 0.76, and the hole expansion ratio (HER) equal to or more than 40%. Therefore, the elongation and the hole expansion ratio can be increased.

Description

Steel sheet having ultra high strength with high formability and method of manufacturing the same

The present invention relates to a steel sheet, and more particularly, to an ultra-high strength and high formability steel sheet having ultra-high strength and improved formability, and a method for manufacturing the same.

In recent years, from the viewpoint of safety and weight reduction of automobiles, high strength of automobile steel sheets is progressing more rapidly. In order to secure passenger safety, steel sheets used as structural members of automobiles must have sufficient impact toughness by increasing the strength or thickness. In addition, sufficient formability is required to be applied to automobile parts, and weight reduction is essential to improve fuel efficiency of automobiles.

In order to realize high strength and formability improvement of steel sheet, CP steel (Complexed Phase steel) composed of martensite, bainite, and ferrite has been developed and produced. In order to secure high strength, CP steel has hardenability by delaying the transformation of ferrite and pearlite in the cold rolling heat treatment step by adding a large amount of Mn, Cr, and B to secure high strength. can However, the CP steel has disadvantages in that it is difficult to realize high yield and poor hole expandability due to the difference in hardness between the soft phase and the low temperature phase. In addition, depending on the part, there is a demand for developing a steel type that requires excellent flangeability and high yield characteristics at the same time.

Korean Patent Application No. 2015-0116788

The technical problem to be achieved by the technical idea of the present invention is to provide an ultra-high strength and high formability steel sheet excellent in ultra-high strength and formability, and excellent elongation and hole expandability, and a method for manufacturing the same.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, an ultra-high strength and high formability steel sheet and a method for manufacturing the same are provided.

According to an embodiment of the present invention, the ultra-high strength and high formability steel sheet, by weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% To 2.7%, Soluble Aluminum (S_Al): 0.01% to 0.10%, Chromium (Cr): 0.1% to 1.0%, Niobium (Nb): 0.01% to 0.05%, Titanium (Ti): 0.001% to 0.05%, Boron (B): 0.0001% to 0.004%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance is iron (Fe) and other unavoidable impurities, has a mixed structure of ferrite, bainite, and martensite, the fraction of ferrite is in the range of 1% to 10%, yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, yield ratio (YR): 0.76 or more, and hole expandability (HER): 40% or more may be satisfied.

According to an embodiment of the present invention, the steel sheet may have a grain size in the range of 1 μm to 10 μm.

According to an embodiment of the present invention, the ultra-high strength and high formability steel sheet, a base steel sheet; and a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the base steel sheet, in weight%, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0% , Manganese (Mn): 2.3% to 2.7%, Soluble Aluminum (S_Al): 0.01% to 0.10%, Chromium (Cr): 0.1% to 1.0%, Niobium (Nb): 0.01% to 0.05%, Titanium (Ti) : 0.001% to 0.05%, boron (B): 0.0001% to 0.004%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% ~ 0.006%, and the balance includes iron (Fe) and other unavoidable impurities, the base steel sheet has a mixed structure of ferrite, bainite, and martensite, and the fraction of ferrite is in the range of 1% to 10% , yield strength (YS): 900 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, yield ratio (YR): 0.76 or more, and hole expansion (HER): 23% or more can be satisfied

According to an embodiment of the present invention, the steel sheet is a hot-dip galvanized steel sheet, and the hot-dip galvanized steel sheet may exhibit a suppression layer coverage in the range of 90% to 99%.

According to an embodiment of the present invention, the steel sheet is an alloyed hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet may exhibit an alloying area fraction in the range of 95% to 99%.

According to an embodiment of the present invention, in the method for manufacturing the ultra-high strength and high formability steel sheet, in weight %, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn) : 2.3% to 2.7%, soluble aluminum (S_Al): 0.01% to 0.10%, chromium (Cr): 0.1% to 1.0%, niobium (Nb): 0.01% to 0.05%, titanium (Ti): 0.001% to 0.05 %, boron (B): 0.0001% to 0.004%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.003%, nitrogen (N): greater than 0% to 0.006%, and The remainder may include: manufacturing a hot-rolled steel sheet using a steel material containing iron (Fe) and other unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature of about 870° C. above the average temperature of A _c3 and A _e3 ; and cooling the cold-rolled steel sheet subjected to the annealing heat treatment at a cooling rate of 3°C to 30°C.

According to an embodiment of the present invention, in the cooling step, the annealing heat-treated steel may be cooled to a cooling temperature of 250 °C ~ M _s -20 °C.

According to an embodiment of the present invention, the manufacturing of the hot-rolled steel sheet comprises the steps of reheating the steel to a temperature of 1,180 °C to 1,220 °C; preparing a rolled material by hot rolling the reheated steel to a finish rolling temperature of 800°C to 960°C; and winding the hot-rolled rolled material at a temperature of 420°C to 650°C.

According to an embodiment of the present invention, the cold-rolled steel sheet subjected to annealing heat treatment has a mixed structure in which ferrite, bainite, tempered martensite and martensite are mixed, and the fraction of ferrite is in the range of 1% to 10%, Yield strength (YS): 900 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, yield ratio (YR): 0.76 or more, and hole expansion (HER): 40% or more can

According to one embodiment of the present invention, the method further includes: immersing the annealed cold-rolled steel sheet in a hot-dip galvanizing bath at a temperature in the range of 450° C. to 500° C. to perform hot-dip galvanizing to prepare a hot-dip galvanized steel sheet. can do.

According to an embodiment of the present invention, the step of producing an alloyed hot-dip galvanized steel sheet by heat-treating the hot-dip galvanized steel sheet at a temperature in the range of 500 °C to 550 °C; may further include.

According to an embodiment of the present invention, in the cooling step, the annealed heat-treated steel may be cooled to a cooling temperature of 400 ℃ ~ B _s -20 ℃.

According to an embodiment of the present invention, the hot-dip galvanized steel sheet has a mixed structure in which ferrite, bainite, and martensite are mixed, the fraction of ferrite is in the range of 1% to 10%, and yield strength (YS) : 900 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, yield ratio (YR): 0.76 or more, and hole expandability (HER): 23% or more.

According to an embodiment of the present invention, the alloyed hot-dip galvanized steel sheet has a mixed structure in which ferrite, bainite, and martensite are mixed, the fraction of ferrite is in the range of 1% to 10%, and the yield strength (YS ): 900 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, yield ratio (YR): 0.76 or more, and hole expandability (HER): 23% or more.

According to the present invention, by reducing the ferrite fraction to 10% or less through the control of the component system and the control of the process conditions and the control of the annealing heat treatment temperature, the yield strength and yield ratio are increased, and the sufficient elongation is ensured, and the hole expandability can increase Accordingly, it is possible to provide an ultra-high strength and high formability steel sheet.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method of manufacturing an ultra-high strength and high formability steel sheet according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

According to the technical idea of the present invention, an ultra-high strength and high formability steel sheet capable of securing a tensile strength of 1180 MPa or more, a high yield ratio of 0.76 or more, and excellent hole expandability through a cold rolling heat treatment process was implemented. The steel sheet may be implemented as a cold rolled steel sheet, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet. In order to secure such a high yield ratio and excellent hole expandability, the ratio of ferrite in the final microstructure of the cold-rolled steel sheet should be less than 10%, and the ratio of composite phases such as martensite, tempered martensite, and bainite can be implemented as a phase of 90% or more. there is.

In order to achieve such a microstructure, it is necessary to adjust the cold rolling heat treatment temperature to suppress ferrite transformation occurring during cooling, and to increase the fraction of low-temperature phases such as bainite and martensite. Specifically, if the annealing heat treatment temperature is higher than the austenite single phase generation temperature, the initial austenite grain size may be increased compared to the case where the heat treatment is performed at a temperature directly above A3 with the same component. As the initial austenite grain size increases, the transformation of a soft phase such as a ferrite phase may be suppressed, while generation of a low-temperature phase such as bainite, martensite, and tempered martensite may be increased.

Hereinafter, an ultra-high strength and high formability steel sheet according to the technical idea of the present invention will be described.

Ultra-high strength and high formability steel sheet according to an embodiment of the present invention, by weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% to 2.7 %, Soluble Aluminum (S_Al): 0.01% to 0.10%, Chromium (Cr): 0.1% to 1.0%, Niobium (Nb): 0.01% to 0.05%, Titanium (Ti): 0.001% to 0.05%, Boron (B) ): 0.0001% to 0.004%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance is iron (Fe ) and other unavoidable impurities.

The role and content of each component included in the ultra-high strength and high formability steel sheet according to the present invention will be described as follows. At this time, the content of the component elements all mean weight % with respect to the entire steel sheet.

Carbon (C): 0.10% to 0.22%

Carbon is an element that contributes to strength. It is dissolved in austenite and causes an increase in the strength of martensite after final cooling, which is effective in increasing material strength. When the carbon content is less than 0.10%, it may be difficult to secure strength. When the carbon content exceeds 0.22%, weldability may deteriorate. Therefore, it is preferable to add the carbon content in an amount of 0.10% to 0.22% of the total weight of the steel sheet.

Silicon (Si): 0.1% to 1.0%

Silicon is a ferrite stabilizing element, and it is dissolved in ferrite to increase ferrite strength and increase dislocation density in ferrite to increase work hardening index. When the content of silicon is less than 0.1%, the solid solution strengthening effect may be reduced, the strength may be lowered, and the elongation may be lowered. When the content of silicon exceeds 1.0%, a silicon-based oxide may be formed on the surface of the steel sheet to make the steel sheet brittle, the hot rolling scale may increase, and surface properties and plating properties may be deteriorated. Therefore, it is preferable to add the content of silicon in an amount of 0.1% to 1.0% of the total weight of the steel sheet.

Manganese (Mn): 2.3% to 2.7%

Manganese as a hardenable element suppresses the formation of the third phase of pearlite and bainite by stabilizing austenite during cooling, thereby facilitating the ideal separation of ferrite and martensite even at a low cooling rate. When the manganese content is less than 2.3%, it may be difficult to secure hardenability and strength. When the manganese content exceeds 2.7%, the quality of the slab may be deteriorated, center segregation may occur, and the bending properties of the final steel sheet may be deteriorated. Therefore, it is preferable to add the manganese content in an amount of 2.3% to 2.7% of the total weight of the steel sheet.

Soluble Aluminum (S_Al): 0.01% to 0.10%

Soluble aluminum is an additive element for the purpose of deoxidation. In addition, in the case of a plating material, aluminum is first oxidized inside the steel sheet due to strong oxidizing power, thereby suppressing the formation of silicon-based oxide in the surface layer portion, thereby improving plating properties. When the content of soluble aluminum is less than 0.01%, the effect of adding soluble aluminum may be insufficient. When the content of soluble aluminum exceeds 0.10%, aluminum inclusions and oxides may be formed, and physical properties and surface quality may be deteriorated. Therefore, it is preferable to add the content of soluble aluminum in an amount of 0.01% to 0.10% of the total weight of the steel sheet.

Chromium (Cr): 0.1% to 1.0%

Chromium is an effective element added to secure strength as a hardenable element. When the content of chromium is less than 0.1%, the effect of adding chromium may be insufficient. When the content of chromium exceeds 1.0%, weldability or heat-affected zone (HAZ) toughness may be reduced, and alloy cost may increase. Therefore, it is preferable to add chromium in an amount of 0.1% to 1.0% of the total weight of the steel sheet.

Niobium (Nb): 0.01% to 0.05%

Niobium combines with carbon (C) and nitrogen (N) contained in steel at high temperatures to form carbides or nitrides, and these niobium-based carbides or nitrides suppress recrystallization and grain growth during rolling to refine grains, thereby strengthening the steel sheet and improve both toughness. When the content of niobium is less than 0.01%, it may be difficult to secure fine precipitates, and it may be difficult to obtain a strengthening effect. When the content of niobium exceeds 0.05%, it may be difficult to obtain a reinforcing effect due to an increase in the carbide size, and cost may increase. Therefore, it is preferable to add niobium in an amount of 0.01% to 0.05% of the total weight of the steel sheet.

Titanium (Ti): 0.001% to 0.05%

Titanium generates Ti(C, N) precipitates with high high-temperature stability, thereby preventing coarsening of austenite grains in the reheating step, thereby improving the toughness of the steel sheet. When the content of titanium is less than 0.001%, it may be difficult to secure strength. When the content of titanium exceeds 0.05%, titanium nitride (TiN) inclusions are coarsened, and properties may be deteriorated. Therefore, titanium is preferably added in an amount of 0.001% to 0.05% of the total weight of the steel sheet.

Boron (B): 0.0001% to 0.004%

Boron can secure low-temperature phase structures such as martensite and bainite, and secure high strength and hardenability. When the content of boron is less than 0.0001%, it may be difficult to secure hardenability. When the content of boron exceeds 0.004%, boron nitride (BN) is generated, and plating adhesion may be deteriorated. Therefore, it is preferable to add boron in an amount of 0.0001% to 0.004% of the total weight of the steel sheet.

Phosphorus (P): >0% to 0.02%

Phosphorus partially contributes to the improvement of strength, but may adversely affect the material by reducing the toughness of the weld joint and the low-temperature impact toughness, and forming fine segregation as well as central segregation. Therefore, the lower the phosphorus content, the better. Therefore, it is preferable to limit the phosphorus to more than 0% to 0.02% (200 ppm) of the total weight of the steel sheet.

Sulfur (S): >0% to 0.003%

Sulfur is an element that is inevitably contained in the manufacture of steel together with phosphorus, and forms non-metallic inclusions such as MnS, and as a low melting point element, the possibility of grain boundary segregation is high, thereby reducing toughness. If the sulfur content exceeds 0.003%, the toughness of the base metal and the weld joint may be greatly reduced. Therefore, it is preferable to limit sulfur to more than 0% to 0.003% (30 ppm) of the total weight of the steel sheet.

Nitrogen (N): >0% to 0.006%

Nitrogen may greatly increase the risk of cracking when playing through AlN formation. Therefore, it is preferable to limit the nitrogen content to more than 0% to 0.006% (60 ppm) of the total weight of the steel sheet.

The remaining component of the steel sheet is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The ultra-high strength and high formability steel sheet formed through the method for manufacturing a steel sheet according to the technical idea of the present invention to be described later by controlling the specific components of the alloy composition and their content ranges described above, cold rolled steel sheet, hot-dip galvanized steel sheet, or alloyed fusion It may be a galvanized steel sheet.

The ultra-high strength and high formability steel sheet, a base steel sheet; and a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on the surface of the base steel sheet. The base steel sheet may be configured to include the above-described components and compositions, and the base steel sheet is, for example, in wt%, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, Manganese (Mn): 2.3% to 2.7%, Soluble Aluminum (S_Al): 0.01% to 0.10%, Chromium (Cr): 0.1% to 1.0%, Niobium (Nb): 0.01% to 0.05%, Titanium (Ti): 0.001% to 0.05%, boron (B): 0.0001% to 0.004%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance may include iron (Fe) and other unavoidable impurities.

The ultra-high strength and high formability steel sheet, yield strength (YS): 900 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 5 or more, and yield ratio (YR): 0.76 or more may be satisfied. The ultra-high strength and high formability steel sheet, yield strength (YS): 900 MPa ~ 1150 MPa, tensile strength (TS): 1180 MPa ~ 1400 MPa, elongation (EL): 5 ~ 13, and yield ratio (YR): 0.76 ~ 0.85 can be satisfied.

The ultra-high strength and high formability steel sheet, in the case of a cold-rolled steel sheet, may satisfy a hole expandability (HER): 40% or more, and may satisfy a hole expandability (HER): 40% to 50%.

On the other hand, when the ultra-high strength and high formability steel sheet is a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the hole expandability (HER): 23% or more may be satisfied, and the hole expandability (HER): 23% to 33% % can be satisfied.

The ultra-high strength and high formability steel sheet may have a mixed structure in which ferrite, bainite, and martensite are mixed. The fraction of ferrite may be, for example, 10% or less, for example, may be in the range of 1% to 10%. The bainite fraction may be, for example, 8% or more, and for example, may be in the range of 8% to 55%. The martensite fraction may be the remaining fraction, for example, may be 20% or more, for example, may be in the range of 20% to 86%. The fraction means an area ratio derived from a microstructure photograph of a steel sheet through an image analyzer.

In addition, the martensite may include tempered martensite. The tempered martensite may appear in the cold-rolled steel sheet, but may not appear in the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet. The tempered martensite fraction may be, for example, 20% or more, for example, may be in the range of 20% to 80%.

The ultra-high strength and high formability steel sheet may include, for example, a microstructure having a grain size of 10 μm or less. That is, the ferrite, bainite, martensite, and tempered martensite may each have a grain size of 10 μm or less, for example, 1 μm to 10 μm.

The ultra-high strength and high formability steel sheet, in the case of a hot-dip galvanized steel sheet, may exhibit, for example, an inhibition layer coverage of 90% or more, for example, in a range of 90% to 99%. The suppression layer coverage means the fraction of Fe-Al compound at the interface between the plating layer and the substrate layer.

The ultra-high strength and high formability steel sheet, in the case of an alloyed hot-dip galvanized steel sheet, may represent, for example, 90% or more, for example, an alloying area fraction in the range of 95% to 99%. The alloying area fraction means a fraction of a zinc-iron alloying portion of the surface.

Hereinafter, with reference to the accompanying drawings will be described with respect to the manufacturing method of the ultra-high strength and high formability steel sheet according to the present invention.

1 is a process flowchart schematically illustrating a method of manufacturing an ultra-high strength and high formability steel sheet according to an embodiment of the present invention.

In the method for manufacturing an ultra-high strength and high formability steel sheet according to the present invention, the semi-finished product to be subjected to the process may be exemplarily a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

Referring to FIG. 1 , the method for manufacturing the ultra-high strength and high formability steel sheet includes a hot-rolled steel sheet manufacturing step (S10); Cold-rolled steel sheet manufacturing step (S20); annealing heat treatment step (S30); cooling step (S40); Hot-dip galvanizing step (S50); and an alloying heat treatment step (S60).

The manufacturing method of the ultra-high strength and high formability steel sheet is, by weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% to 2.7%, soluble aluminum (S_Al): 0.01% to 0.10%, Chromium (Cr): 0.1% to 1.0%, Niobium (Nb): 0.01% to 0.05%, Titanium (Ti): 0.001% to 0.05%, Boron (B): 0.0001% to 0.004%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable manufacturing a hot-rolled steel sheet using a steel material containing impurities (S10); manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet (S20); annealing the cold-rolled steel sheet at a temperature of 870° C. or higher than the average temperature of A _c3 and A _e3 (S30); and cooling the cold-rolled steel sheet subjected to the annealing heat treatment at a cooling rate of 3° C. to 30° C. (S40). Accordingly, it is possible to manufacture a cold rolled steel sheet subjected to annealing heat treatment. The annealed cold rolled steel sheet may have a mixed structure of martensite, tempered martensite, bainite, and ferrite.

In addition, the method for manufacturing the ultra-high strength and high formability steel sheet may further include a step (S50) of immersing the annealed heat-treated cold-rolled steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing to produce a hot-dip galvanized steel sheet (S50). there is. The hot-dip galvanized steel sheet may have a mixed structure in which martensite, bainite, and ferrite are mixed.

In addition, the method of manufacturing the ultra-high strength and high formability steel sheet, the step (S60) of producing an alloyed hot-dip galvanized steel sheet by alloying heat treatment of the hot-dip galvanized steel sheet; further includes. The alloyed hot-dip galvanized steel sheet may have a mixed structure in which martensite, bainite, and ferrite are mixed.

Hereinafter, the manufacturing method of the ultra-high strength and high formability steel sheet of the present invention will be described in detail step by step.

Hot-rolled steel sheet manufacturing step (S10)

The hot-rolled steel sheet manufacturing step (S10) is a step of manufacturing a hot-rolled steel sheet by hot-rolling a steel material having the above-described composition.

The manufacturing of the hot-rolled steel sheet may include reheating the steel to a reheating temperature at, for example, a Slab Reheating Temperature (SRT) of 1,180°C to 1,220°C; preparing a rolled material by hot rolling the reheated steel to a finish delivery temperature (FDT) of 800° C. to 960° C.; and cooling the hot-rolled rolled material at a cooling rate of 10° C./sec to 30° C./sec and winding it at a coiling temperature (CT) of 420° C. to 650° C.; may include.

When the reheating temperature is less than 1,180°C, a problem in which the hot rolling load is rapidly increased may occur. When the reheating temperature exceeds 1,220°C, the amount of surface scale may increase, leading to loss of material, and energy may be wasted.

When the finish rolling temperature is less than _Ar3 , for example, less than 800 ℃, productivity may be reduced due to an increase in the rolling reduction during hot rolling. When the finish rolling temperature exceeds 960° C., the initial grain size may be coarsened, and thus the strength and bendability of the cold-rolled product may be reduced.

When the coiling temperature is less than 420 ° C., the strength of the hot-rolled steel sheet increases and productivity decreases due to an increase in the reduction ratio during cold rolling, and the productivity of the subsequent process may be lowered due to generation of coils due to heat transformation. When the coiling temperature exceeds 650° C., surface defects may occur due to surface color difference.

The hot-rolled steel sheet manufactured in this way may have a mixed structure in which martensite, bainite, and ferrite are mixed.

Cold-rolled steel sheet manufacturing step (S20)

The cold-rolled steel sheet manufacturing step (S20) is a step of manufacturing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet. Before performing cold rolling, pickling treatment of washing the hot-rolled steel sheet with acid may be performed. Next, the pickling-treated hot-rolled steel sheet is subjected to cold rolling at an average reduction ratio of 30% to 70%, for example, to form a cold-rolled steel sheet. As the average reduction ratio is higher, there is an effect of increasing the formability due to the effect of refining the tissue. When the average reduction ratio is less than 40%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 70%, the force of the rolling roll may be increased to increase the process load.

Annealing heat treatment step (S30)

The annealing heat treatment step (S30) is a step of annealing the cold rolled steel sheet in a continuous annealing furnace having a normal slow cooling section. The annealing heat treatment may be performed, for example, at a temperature of at least the average temperature of A _c3 and A _e3 to 870°C, for example, at a temperature of 814°C to 870°C. The annealing heat treatment may be performed at a temperature higher than the average temperature of A _c3 and A _e3 (= (A _c3 +A _e3 )/2) in order to secure the austenite single phase region. The average temperature of A _c3 and A _e3 may vary depending on the composition. Therefore, when the annealing heat treatment is performed at a temperature lower than the average temperature of A _c3 and A _e3 , the austenite single-phase region may not be sufficiently secured. When the annealing heat treatment is performed at a temperature exceeding 870° C., the austenite grains may be coarsened and the strength may be reduced.

Here, A _c3 and A _e3 may be defined as follows, respectively.

A _c3 (℃) = 910 - 203 C ^0.5 + 44.7 Si + 31.5 Mo - 15.2 Ni +104 V + 13.1 W

A _e3 (℃) = Calculation result from commercial Thermo-Calc TCFE 9 database

The annealing heat treatment may be performed, for example, for 50 seconds to 250 seconds. When the annealing heat treatment is performed for less than 50 seconds, sufficient cracks may not occur. When the annealing heat treatment is performed for more than 250 seconds, the austenite grains may be coarsened and the strength may be reduced.

Cooling step (S40)

The cooling step (S40) is a step of cooling the annealed cold-rolled steel sheet. The cooling step may be performed at a cooling rate of 3°C to 30°C. The cooling step may be cooled to various cooling temperatures. The temperature may be controlled to secure a low temperature phase of 90% or more, and the cooling temperature may be changed according to the process.

In the case of manufacturing a cold-rolled steel sheet, the annealed heat-treated steel may be cooled, for example, to a cooling temperature of 250° C. to M _s -20° C., for example, to a cooling temperature of 250° C. to 350° C. If the cooling temperature is less than 250 ℃, the fraction of martensite may be increased to reduce the target elongation. When the cooling temperature exceeds M _s -20°C, the target strength cannot be obtained.

Here, M _s is a martensitic transformation start temperature, and may be defined as follows, respectively.

M _s (℃) = 656 - 57.7 C - 35 Mn - 75 Si - 15 Ni - 34 Cr - 41 Mo

On the other hand, in the case of manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the annealed heat-treated steel is, for example, at a cooling temperature of 400° C. to B _s -20° C., for example, 400° C. to 550° C. It can be cooled to a cooling temperature. The range of the cooling temperature may be required for the subsequent process, maintaining the temperature of the plating bath and forming bainite.

Here, B _s is a bainite transformation starting temperature, and may be defined as follows, respectively.

B _s (℃) = 539 - 423 C - 30.4 Mn - 17.7 Ni - 12.1 Cr - 7.5 Mo

The cold-rolled steel sheet manufactured as described above may have a mixed structure in which martensite, tempered martensite, bainite, and ferrite are mixed.

Hot-dip galvanizing step (S50)

The hot-dip galvanizing step (S50) is a step of forming a hot-dip galvanizing layer on the cold-rolled steel sheet by immersing the annealed cold-rolled steel sheet in a hot-dip galvanizing bath. The temperature of the plating bath may be 450°C to 500°C depending on the type and ratio of alloying elements constituting the plating layer, and the composition of the base material (cold-rolled steel sheet). While the hot-dip galvanized layer is easily formed on the surface of the cold-rolled steel sheet under the plating bath conditions, the adhesion of the plating layer may be excellent. If necessary, the hot-dip galvanized steel sheet may be manufactured by cooling it to room temperature at a cooling rate of 1° C./sec to 100° C./sec.

The hot-dip galvanized steel sheet manufactured in this way may have a mixed structure in which martensite, bainite, and ferrite are mixed.

alloying heat treatment step (S60)

The alloying heat treatment step (S60) is a step of performing alloying heat treatment by maintaining the hot-dip galvanized steel sheet at a temperature in the range of, for example, 500° C. to 550° C. for, for example, 10 seconds to 60 seconds. The alloying heat treatment step may be performed continuously without cooling the hot-dip galvanized steel sheet manufactured after performing the previous hot-dip galvanizing step. During the alloying heat treatment under the above temperature conditions, the hot-dip galvanized layer may be stably grown, and the adhesion of the plating layer may be excellent. When the alloying heat treatment temperature is less than 500 ℃, alloying may not proceed sufficiently, the soundness of the hot-dip galvanizing layer may be reduced. When the alloying heat treatment temperature exceeds 550 °C, the material may change as the temperature goes to an abnormal temperature section. Then, by cooling to room temperature, an alloyed hot-dip galvanized steel sheet can be manufactured.

The alloyed hot-dip galvanized steel sheet manufactured as described above may have a mixed structure in which martensite, bainite, and ferrite are mixed.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Steel materials having the composition (unit: weight %) shown in Table 1 below were prepared, and cold rolled steel sheets (CR) according to Examples and Comparative Examples were manufactured through predetermined hot rolling and cold rolling processes. In addition, if necessary, hot-dip galvanizing alloy (GI) was prepared by performing hot-dip galvanizing, and alloying heat treatment was performed to prepare an alloying hot-dip galvanizing alloy (GA). Accordingly, an ultra-high strength and high formability steel sheet was manufactured.

The remainder in Table 1 is iron (Fe) and other unavoidable impurities. For reference, the content of boron (B) and nitrogen (N) is expressed in ppm.

steel grade C Si Mn S_Al Cr Nb Ti P S B N W 0.121 0.214 2.60 0.029 0.41 0.021 0.019 0.011 0.0019 19 37 X 0.123 0.098 2.50 0.050 0.58 0.042 0.025 0.013 0.0019 21 42 Y 0.158 0.435 2.41 0.025 0.17 0.010 0.044 0.013 0.0021 23 34 Z 0.203 0.735 2.42 0.048 0.37 0.018 0.023 0.014 0.0021 24 40

Table 2 shows various temperatures determined according to the composition of the steel type in Table 1.

steel grade A _c3
(℃) A _e3
(℃) (A _c3 + A _e3 )/2
(℃) B _s
(℃) M _s
(℃) W 849 787 818 528 404 X 843 785 814 534 404 Y 849 794 821.5 524 397 Z 851 789 820 492 375

Here, A _c3 temperature, A _e3 temperature, B _s temperature, and M _s temperature can be defined as: A _c3 (°C) = 910 - 203 C ^0.5 + 44.7 Si + 31.5 Mo - 15.2 Ni +104 V + 13.1 W

A _e3 (℃) = Calculation result from commercial Thermo-Calc TCFE 9 database

B _s (℃) = 539 - 423 C - 30.4 Mn - 17.7 Ni - 12.1 Cr - 7.5 Mo

M _s (℃) = 656 - 57.7 C - 35 Mn - 75 Si - 15 Ni - 34 Cr - 41 Mo

Table 3 shows the manufacturing process conditions of Examples and Comparative Examples of the present invention.

division steel grade steel
division FDT
(℃) CT
(℃) Annealing temperature
(℃) cooling temperature
(℃) Comparative Example 1 W CR 880 520 800 290 Example 1 W CR 880 520 820 290 Comparative Example 2 W GA 900 500 815 450 Example 2 W GA 900 500 840 450 Example 3 W GI 900 620 830 450 Comparative Example 3 X CR 860 560 800 300 Example 4 X CR 860 560 830 230 Example 5 X GI 900 560 840 500 Comparative Example 4 Y GA 920 520 815 480 Example 6 Y GA 920 520 830 480 Comparative Example 5 Y GI 880 520 800 490 Example 7 Z CR 960 520 820 300 Example 8 Z GA 900 520 820 460

In Table 3, "FDT" is the finish rolling temperature at the time of hot rolling, and "CT" is the coiling temperature at the time of hot rolling. In addition, "CR" is annealed cold-rolled steel sheet, "GI" is hot-dip galvanized steel sheet, and "GA" is alloyed hot-dip galvanized steel sheet. Referring to Table 3, Comparative Examples 1 and 2 are steel grade W In the case of annealing heat treatment at 800 °C and 815 °C lower than the lower limit of the annealing heat treatment temperature of 818 °C. Comparative Example 3 is a case of annealing heat treatment at 800° C. lower than the lower limit of 814° C. of the annealing heat treatment temperature of steel type X. Comparative Examples 4 and 5 are cases of annealing heat treatment at 815° C. and 800° C. lower than the lower limit of 821.5° C. of the annealing heat treatment temperature of the steel grade Y. The cooling temperature of Comparative Examples satisfies the scope of the present invention.

Table 4 shows the fractions of microstructures of Examples and Comparative Examples of the present invention.

division steel grade steel
division bainite
(%) marten
site
(%) tempered
martensite
(%) ferrite
(%) Comparative Example 1 W CR 7 3 73 15 Example 1 W CR 8 5 80 7 Comparative Example 2 W GA 40 48 - 12 Example 2 W GA 43 51 - 6 Example 3 W GI 45 49 - 6 Comparative Example 3 X CR 7 3 72 18 Example 4 X CR 8 4 79 9 Example 5 X GI 52 40 - 8 Comparative Example 4 Y GA 40 46 - 14 Example 6 Y GA 39 54 - 7 Comparative Example 5 Y GI 35 48 - 17 Example 7 Z CR 10 10 76 4 Example 8 Z GA 40 58 - 2

Referring to Table 4, all of the comparative examples have a ferrite fraction exceeding 10%, which is a value exceeding the upper limit of the present invention. Since such ferrite is a soft phase, the strength may be reduced, and also the yield ratio may be reduced. On the other hand, in the Examples, the fraction of ferrite was all less than 10%. Table 5 shows the mechanical properties of Examples and Comparative Examples of the present invention.

division steel grade steel
division yield strength
(MPa) The tensile strength
(MPa) yield ratio
elongation
(%) Hall expandability
(%) Comparative Example 1 W CR 813 1241 0.65 9.1 29.2 Example 1 W CR 1039 1277 0.81 7.0 43.2 Comparative Example 2 W GA 821 1255 0.65 10.6 19.1 Example 2 W GA 1052 1280 0.79 8.7 24.5 Example 3 W GI 1021 1261 0.81 9.0 25.3 Comparative Example 3 X CR 790 1156 0.66 13.2 27.6 Example 4 X CR 920 1191 0.77 11.5 41.2 Example 5 X GI 978 1201 0.81 10.1 26.3 Comparative Example 4 Y GA 862 1230 0.70 9.7 19.8 Example 6 Y GA 1093 1298 0.84 7.8 23.7 Comparative Example 5 Y GI 864 1243 0.69 9.2 18.7 Example 7 Z CR 1100 1326 0.82 5.6 45.6 Example 8 Z GA 1137 1351 0.84 6.1 24.1

Referring to Table 5, it was found that the yield strength of all comparative examples did not reach 900 MPa, which is the lower limit of the yield strength range of the present invention. In addition, it was found that the yield ratio of all Comparative Examples did not reach 0.76, which is the lower limit of the range of the yield ratio of the present invention. In the case of hole expandability, Comparative Examples 1 and 3, which are cold-rolled steel sheets, are the lower limits of the range of the hole expandability of the present invention. was found to be less than 40%. In addition, Comparative Example 4, which is an alloyed hot-dip galvanized steel sheet, and Comparative Example 5, which is a hot-dip galvanized steel sheet, did not reach the lower limit of the hole expandability range of the present invention, which is 23%.

On the other hand, in the case of the examples, yield strength (YS): 900 MPa to 1150 MPa, tensile strength (TS): 1180 MPa to 1400 MPa, elongation (EL): 5 to 13, and yield ratio ( YR): 0.76 to 0.85 were satisfied. In addition, in the examples, in the case of the cold-rolled steel sheet, the hole expandability (HER) within the scope of the present invention: 40% or more was satisfied. In the examples, in the case of a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the hole expandability (HER) of the present invention: 23% or more was satisfied. This is analyzed to be secured by controlling the fraction of ferrite, which is a soft phase, to 10% or less.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Accordingly, the scope of the present invention should be judged by the claims set forth below.

Claims (14)

By weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% to 2.7%, soluble aluminum (S_Al): 0.01% to 0.10%, chromium (Cr): 0.1% to 1.0%, niobium (Nb): 0.01% to 0.05%, titanium (Ti): 0.001% to 0.05%, boron (B): 0.0001% to 0.004%, phosphorus (P): 0% more than 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance contains iron (Fe) and other unavoidable impurities;
It has a mixed structure of ferrite, bainite, and martensite,
The fraction of ferrite is in the range of 1% to 10%,
Yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, Elongation (EL): 5 or more, Yield ratio (YR): 0.76 or more, and hole expansion (HER): 40% or more doing,
Super high strength and high formability steel sheet. The method of claim 1,
The steel sheet has a grain size in the range of 1 μm to 10 μm,
Super high strength and high formability steel sheet. base steel plate; and
Including; a hot-dip galvanized layer or alloyed hot-dip galvanized layer formed on the surface of the base steel sheet;
The base steel sheet is, by weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% to 2.7%, soluble aluminum (S_Al): 0.01% ~ 0.10%, Chromium (Cr): 0.1% ~ 1.0%, Niobium (Nb): 0.01% ~ 0.05%, Titanium (Ti): 0.001% ~ 0.05%, Boron (B): 0.0001% ~ 0.004%, Phosphorus ( P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.003%, nitrogen (N): more than 0% to 0.006%, and the balance contains iron (Fe) and other unavoidable impurities;
The base steel sheet has a mixed structure of ferrite, bainite, and martensite,
The fraction of ferrite is in the range of 1% to 10%,
Yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, Elongation (EL): 5 or more, Yield ratio (YR): 0.76 or more, and hole expansion (HER): 23% or more doing,
Super high strength and high formability steel sheet. 4. The method of claim 3,
The steel sheet is a hot-dip galvanized steel sheet,
The hot-dip galvanized steel sheet exhibits a suppression layer coverage in the range of 90% to 99%,
Super high strength and high formability steel sheet. 4. The method of claim 3,
The steel sheet is an alloyed hot-dip galvanized steel sheet,
The alloyed hot-dip galvanized steel sheet represents an alloying area fraction in the range of 95% to 99%,
Super high strength and high formability steel sheet. By weight, carbon (C): 0.10% to 0.22%, silicon (Si): 0.1% to 1.0%, manganese (Mn): 2.3% to 2.7%, soluble aluminum (S_Al): 0.01% to 0.10%, chromium (Cr): 0.1% to 1.0%, niobium (Nb): 0.01% to 0.05%, titanium (Ti): 0.001% to 0.05%, boron (B): 0.0001% to 0.004%, phosphorus (P): 0% Excess ~ 0.02%, Sulfur (S): Exceeding 0% ~ 0.003%, Nitrogen (N): Exceeding 0% ~ 0.006%, and the remainder using steel containing iron (Fe) and other unavoidable impurities. manufacturing;
manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet;
annealing the cold-rolled steel sheet at a temperature of about 870° C. above the average temperature of A _c3 and A _e3 ; and
Including; cooling the annealing heat-treated cold-rolled steel sheet at a cooling rate of 3°C to 30°C;
A method for manufacturing an ultra-high strength and high formability steel sheet. 7. The method of claim 6,
In the cooling step,
Cooling the annealing heat treated steel to a cooling temperature of 250 ℃ ~ M _s -20 ℃,
A method for manufacturing an ultra-high strength and high formability steel sheet. 7. The method of claim 6,
The step of manufacturing the hot-rolled steel sheet,
reheating the steel to a temperature of 1,180° C. to 1,220° C.;
preparing a rolled material by hot rolling the reheated steel to a finish rolling temperature of 800°C to 960°C; and
Including; winding the hot-rolled rolled material at a temperature of 420 ℃ ~ 650 ℃;
A method for manufacturing an ultra-high strength and high formability steel sheet. 7. The method of claim 6,
The cold-rolled steel sheet subjected to the annealing heat treatment,
It has a mixed structure of ferrite, bainite, tempered martensite and martensite, and the fraction of ferrite is in the range of 1% to 10%,
Yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, Elongation (EL): 5 or more, Yield ratio (YR): 0.76 or more, and hole expansion (HER): 40% or more doing,
A method for manufacturing an ultra-high strength and high formability steel sheet. 7. The method of claim 6,
Further comprising; immersing the annealed cold-rolled steel sheet in a hot-dip galvanizing bath at a temperature in the range of 450° C. to 500° C. to perform hot-dip galvanizing to prepare a hot-dip galvanized steel sheet;
A method for manufacturing an ultra-high strength and high formability steel sheet. 11. The method of claim 10,
Further comprising; further comprising: alloying heat treatment of the hot-dip galvanized steel sheet at a temperature in the range of 500 °C to 550 °C to produce an alloyed hot-dip galvanized steel sheet;
A method for manufacturing an ultra-high strength and high formability steel sheet. 11. The method of claim 10,
In the cooling step,
Cooling the annealing heat-treated steel to a cooling temperature of 400 ℃ ~ B _s -20 ℃,
A method for manufacturing an ultra-high strength and high formability steel sheet. 11. The method of claim 10,
The hot-dip galvanized steel sheet,
It has a mixed structure in which ferrite, bainite, and martensite are mixed, and the fraction of ferrite is in the range of 1% to 10%,
Yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, Elongation (EL): 5 or more, Yield ratio (YR): 0.76 or more, and hole expansion (HER): 23% or more doing,
A method for manufacturing an ultra-high strength and high formability steel sheet. 12. The method of claim 11,
The alloyed hot-dip galvanized steel sheet,
It has a mixed structure in which ferrite, bainite, and martensite are mixed, and the fraction of ferrite is in the range of 1% to 10%,
Yield strength (YS): 900 MPa or more, Tensile strength (TS): 1180 MPa or more, Elongation (EL): 5 or more, Yield ratio (YR): 0.76 or more, and hole expansion (HER): 23% or more doing,
A method for manufacturing an ultra-high strength and high formability steel sheet.

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